1. Field of the Invention
The invention relates generally to an optical amplifier, and more particularly to, an optical amplifier to which holmium is doped.
2. Description of the Prior Art
In order to utilize the entire range of 1200˜1700 nm being a low-loss wavelength region of a silica optical fiber for transmission, it is inevitably required that a wide-band optical amplifier be developed. An ion doped optical fiber amplifier that operates at a 1300 nm band, a 1450 nm band and a 1530˜1610 nm band has been developed so far. It was found that fluorescence of 1620˜1690 nm band is emitted from a holmium (Ho) doped amorphous material. However, an example implementing a laser or am amplifier of any type using it has not yet been reported. (Reference: T. Schweizer, B. N. Samson, J. R. Hector, W. S. Brocklesby, D. W. Hewak, D. N. Payne, “Infrared Emission from Holmium Doped Gallium Lanthanum Sulfide Glass” Infrared Physics & Technology Vol. 40, (1999) pp. 329-335).
Fluorescence of the 1620˜1690 nm band is generated from Ho3+:5I5→5I7 transition. The 5I5 level being an upper level is approximately located at a 900 nm band. If it is pumped in this wavelength, however, excited state absorption of the pump is caused to excite holmium ions to the 5F1 level. Thus, in order to increase the excitation efficiency of 1620˜1690 nm fluorescence, it is required that the wavelength in which the excited state absorption of the pump light is generated be omitted. (Reference: Y. G. Choi, B. J. Park, K. H. Kim, “Ho3+: (5S2, 5F4)→5I5 Transition in Fluoride Glasses”, Chemical Physics Letters, vol. 354, (2002) pp. 69-74).
Further, as the fluorescence lifetime of the 5I7 level being a lower level is longer than that of the upper level, inversion of the population between the two levels is difficult. This degrades the operating characteristics of the laser or the amplifier. Therefore, a method of efficiently accomplishing inversion of the population between the two levels must be found. The depopulation rate could be faster by forming a cavity utilizing a corresponding lower level as a lasing level in order to lower the population density of the lower level. For example, there is a previous experimental result example that in order to implement a laser using the Ho3+: 5I5→5I6 transition, lasing was achieved at a 1.2 μm wavelength band corresponding to the energy between the 5I6 level and the 5I8 level being the ground level, so that inversion of the population between the 5I5 level and the 5I6 level could be accomplished (Reference: J. Schneider, C. Carbonnier, U. B. Unrau, “Characterization of a Ho3+-Added Fluoride Fiber Laser with a 3.9-μm Emission Wavelength”, Applied Optics, vol. 36, no. 33, (1997) pp. 8595-8600). Therefore, in case of 1.6 μm fluorescence of holmium, lasing may be caused at a 2.0 μm wavelength band corresponding to the energy between the 5I7 level and the ground 5I8 level.